1. Field of the Invention
The present invention relates to a system for changing the parity structure of data in a shared storage device system and, in particular, altering the parity structure of a RAID array.
2. Description of the Related Art
In Redundant Arrays of Independent Disks (RAID) systems, data files and related parity are striped across multiple disk drives. In storage subsystems which manage numerous hard disk drives as a single direct access storage device (DASD), the RAID logic is implemented in the controller of the subsystem. RAID storage methodologies have also been implemented in software for execution on a single host computer. This allows the single host computer, such as a personal computer, to implement RAID storage techniques on local hard disk drive space. Such software RAID methodologies are described in "Algorithms for Software and Low Cost Hardware RAIDs," by Jai Menon, Jeff Reigel, and Jim Wyllie, Document No. 1063-6390/95, pgs. 411-418 (IEEE 1995).
One technique for altering the arrangement of logical drives in a storage subsystem requires that the RAID array to be taken off-line to back-up the data in order to delete the logical drive or array, create new logical drives/arrays, format the storage space, and then, finally, restore the data. The PC ServeRAID SCSI adaptor from International Business Machines Corporation ("IBM.RTM."), the assignee of the present patent application, offers logical drive migration (LDM). "IBM" is a registered trademark of IBM, and "ServeRAID" is a trademark of IBM. The LDM feature allows a single IBM PC ServeRAID SCSI adaptor to alter the structure of a RAID array by altering the RAID level or adding or removing a storage device, e.g., hard disk drive, to increase or decrease the size of the current array. Integrating one or more new hard disks into an existing array increases the size of all logical drives in the array. The IBM PC ServeRAID SCSI adaptor can concurrently service input/output requests to data in the RAID array involved in logical drive migration (LDM) activities, i.e., having their RAID level changed or distributing the current RAID level across a different number of disk drives.
As networks increase in size, an adaptor can be overburdened with I/O requests to a storage device managed by the adaptor. Moreover, if the IBM PC ServeRAID SCSI adaptor is performing logical drive migration (LDM) activities, the adaptor will take longer to service I/O requests to the storage device.